This invention relates to variable reluctance motors, and more particularly to single-phase variable reluctance motors particularly suited for use as fan motors or in other low starting torque applications.
In any simple magnetic circuit, consisting of a fixed and a movable flux-carrying element, the movable element will tend to assume a position such that the reluctance of the magnetic circuit is a minimum. A reluctance force is exerted on the movable piece to try to make it assume this position of minimum reluctance.
In a variable reluctance motor, both the fixed element (the stator) and the movable element (the rotor) are salient, having teeth on both the stator and the rotor, with phase windings only on the stator poles. As a result, the variable reluctance motor, and particularly the single-phase variable reluctance motor has a simple and relatively inexpensive construction.
The design is not, however, without its disadvantages. For example, starting torque for these machines is low. But that is not a problem for low torque applications such as fans. One more significant problem with single-phase variable reluctance motors is that the rotor may assume a position at rest from which it is impossible to start the motor. This position is, of course, the minimum reluctance position.
U.S. Pat. No. 4,616,165 to Compter addresses this problem by providing a pair of off-center permanent magnets adjacent the rotor to ensure that the rotor always assumes a preferred stationary position from which starting in a desired direction is possible. The motor of this patent, however, could be improved. The two permanent magnets add complexity to the motor. Moreover, the asymmetrical placement of these magnets has complex effects on the torque applied to the rotor.